Question: Christopher is 2 times as old as Stephanie. 28 years ago, Christopher was 6 times as old as Stephanie. How old is Stephanie now?
Solution: We can use the given information to write down two equations that describe the ages of Christopher and Stephanie. Let Christopher's current age be $c$ and Stephanie's current age be $s$ The information in the first sentence can be expressed in the following equation: $c = 2s$ 28 years ago, Christopher was $c - 28$ years old, and Stephanie was $s - 28$ years old. The information in the second sentence can be expressed in the following equation: $c - 28 = 6(s - 28)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $s$ , it might be easiest to use our first equation for $c$ and substitute it into our second equation. Our first equation is: $c = 2s$ . Substituting this into our second equation, we get: $2s$ $-$ $28 = 6(s - 28)$ which combines the information about $s$ from both of our original equations. Simplifying the right side of this equation, we get: $2 s - 28 = 6 s - 168$ Solving for $s$ , we get: $4 s = 140.$ $s = 35$.